EP patent 548 187 discloses a sampling method and assembly for taking representative samples from pressurized fluid systems, particularly hydraulic or lubricating oil systems. In some cases, however, the patented sampler may not be able to secure a completely representative fluid sample. In particular, this is so in cases where it is desirable to analyse the gas contents of a fluid. An example is samples from insulation oil in a transformer which, when it is old and worn, loses its insulation properties, resulting in a danger of explosion caused by sparks that may occur owing to the insulation defects. Such sparks result in the formation of free acetylene and hydrogen which could be detected in a gas chromatograph. The latter gas, in particular, appears in very low concentrations and is very elusive. Thus, a prerequisite for obtaining a correct result from an analysis that accurately indicates the amount of hydrogen in the sample is that no gas escapes during the sampling operation. Similarly, when sampling fluids included in processes of the food industry or pharmaceutic or medical processes, it is most important that sterile samples are not contaminated by micro organisms. In such cases the fluid sample would normally be tapped directly into a sampling bottle which is then sealed by a fluid-tight membrane. Also in this case there is the danger that the sample fluid be contaminated when transferred to the sample bottle.